Residual contamination of sediments and groundwater by uranium milling operations presents a lingering problem at former mill sites throughout the upper Colorado River Basin. The U.S. Department of Energy Old Rifle, Colorado former mill site serves as a natural laboratory for investigating the underlying causes of uranium plume persistence and potential remediation strategies. Long-term research activities have yielded fundamental insights into the factors controlling uranium mobility over episodic to decadal timescales including strong couplings between climate, hydrology, microbiology, geochemistry, and mineralogy. Recent studies have focused on the role that surface water-groundwater interactions play in sustaining uranium delivery to the aquifer. Annual snowmelt-driven increases in Colorado River discharge induce 1-2 m excursions in groundwater elevation at the site, which enables residual tailings-contaminated materials to become hydrologically connected to the aquifer for short periods of time during peak discharge. The episodic contact between shallow groundwater and residual contamination leads to abrupt 20-fold increases in groundwater uranium concentration, which serve to seasonally replenish the plume. Uranium isotope composition changes abruptly as uranium concentrations increase reflecting the contribution of a temporally distinct contaminant reservoir. Superimposed upon these natural drivers of plume persistence have been experimental manipulations designed to alter the mobility of uranium in groundwater under controlled conditions wherein deliberate changes to groundwater chemistry are used to alter microbial activity, redox conditions, and uranium speciation and sorption. Recent experiments have focused on inducing precipitation of hydroxyapatite within the contaminated aquifer to create a strong sink for uranium in its native hexavalent form via sorption to and incorporation within the apatite structure thereby removing it from solution. Three single day injections over the course of 4-weeks have led to rapid and persistent decreases in uranium concentrations to low parts per trillion levels with sustained removal exceeding one year post-injection. These results contrast substantially to those obtained through site experiments designed to stimulate enzymatic uranium reduction and removal as insoluble tetravalent uranium where sustained injectate delivery was required to maintain effective uranium removal for long periods of time.

Drivers of and solutions for uranium plume persistence at DOE’s Old Rifle, Colorado former uranium mill site

2019 Spring National Meeting

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